The external quantum efficiency of a light emitting diode, herein referred to as LED, can be defined as:ηext=γ·ηcap·ηint·ηout,where γ is the injection efficiency, ηcap is the efficiency of carrier capture into light generation region, ηint is the radiative efficiency provided by radiative recombination of carriers inside the light generation region, ηout is the light extraction efficiency. Maximum LED efficiency is obtained by maximizing all these parameters. The first three factors are overlapping and it is optimal to address them simultaneously during design of a LED structure.
The first factor to address is the efficiency of carrier capture into light generation region. In case of lattice mismatch between the device layers, thickness of the layer where electrons and holes recombine should be small enough to preserve material quality and avoid strain relaxation via nucleation of dislocations. However carrier capture into the light generation layer is significantly reduced when decreasing the layer thickness. The capture can be characterized by the relation
                    q        +            -              q        -              =                  -        n            ⁢                          ⁢              d        τ              ,where q+ and q− are the carrier fluxes, incident to the light generation layer and transmitted over it, n is the carrier concentration in an emitter, d is the width of the light generation narrow bandgap layer, and τ is the capture time, usually determined by electron-optical phonon interaction. The relation shows that captured part of the carrier flux is decreased with decreasing the width of the light generation layer and is reverse proportional to the capture time. For fixed width of the light generation layer, capture of carriers is less effective for electrons than for holes due to lower value of electron effective mass and, consequently, longer energy relaxation time τ. Another inherent mechanism, which reduces carrier capture into the narrow bandgap region, is partial reflection of the electron or hole wave from that region. Therefore, the probability for carrier to be inside the narrow bandgap region decreases, which results in less effective coupling with localized state inside narrow bandgap layer and in increasing of the capture time τ. In result, the capture efficiency of the device drops and maximum of the efficiency ηext vs. injection current is located far below the typical device operating current. A number of structures were proposed to solve this problem. One of the solutions is to use an additional wide bandgap layer on the hole injecting side of the light generation region to prevent electron overflow over this region. This solution was adapted to the case of nitride-based light emitting devices by Nagahama et al in U.S. Pat. No. 6,677,619 and references therein. However, presence of such barrier increases electron and hole reflection, making this solution not optimal. To prevent carriers escape from the light generation region, Nakamura et al in US patent application No 2004/0101012 suggested to insert two barriers from both sides of the light generation region. Due to in general this solution leads to high reflection of carriers from barriers, the authors suggested to make the barriers as thin as possible to increase probability of carrier tunneling through them. One disadvantage of this solution, however, is that the tunneling of carriers in this case is not resonant, and therefore for any reasonable barrier thickness the reflection of carriers from the barriers significantly decreases efficiency of carrier capture into the light generation region. Wang et al in GB patent No 2,352,326 disclose the CART structure, where electrons are collected in a preliminary reservoir in n-type part of structure, from which they resonantly tunnel to the light generation region. The reservoir should be thick enough to effectively collect carriers. In fact, it is hard to realize thick high quality layer on the basis of lattice mismatched semiconductor materials.
The second factor to address is the injection efficiency. Since light is generated inside a thin light generation layer, which is placed near the p-n interface, it is desirable to provide maximum injection efficiency at this interface. Ordinary solution is to dope emitters as large as possible without deterioration of material quality. However, for the materials where the concentration of active doping centers in one of the emitters is limited by fundamental material properties, excessive doping of the other emitter breaks the balance of electron and hole injection currents at the light generation layer resulting in reduced injection efficiency.
The third factor to address is the radiative efficiency. If a device is made from pyroelectric materials, spontaneous polarization and piezo polarization induced by strain exist in the structure, giving rise to built-in electric field, which spatially separates electrons and holes inside the light generation region. In result, non-direct optical transitions are required for radiative recombination. Such non-direct recombination leads to a decrease of device radiative quantum efficiency. The phenomenon is discussed in a number of publications, including Bernardini et al, “Spontaneous polarization and piezoelectric constants of III-V nitrides”, American Physical Society Journal, Physics Review B, Vol. 56, No. 16, 1997, pages R10024-R10027; Takeuchi et al, “Quantum-Confined Stark Effect due to Piezoelectric Fields in GaInN Strained Quantum Wells”, Japanese Journal of Applied Physics, Vol. 36, Part 2, No. 4, 1997, pages L382-L385; and Ambacher et al, “Pyroelectric properties of Al(In)GaN/GaN hetero- and quantum well structures”, Journal of Physics: Condensed Matter, Vol. 14, 2002, pages 3399-3434. To some extent the effect of built-in polarization can be minimized by use of very thin light generation layers. However, such small thickness, as we mentioned above, results in non-effective carrier capture. Besides, width of the light generation layers might become comparable with thickness fluctuations. These fluctuations can lead to formation of “holes” in the light generation layers, which act as non-radiative recombination centers, thus additionally reducing device efficiency. As a conclusion, the polarization induced built-in electric field limits both radiative efficiency and capture rate. Ibbetson et al in U.S. Pat. No. 6,515,313 disclose several techniques to diminish the effect of polarization induced charges: selective doping to provide an impurity charge, which might compensate polarization induced charge; cladding layers with graded composition; active region with graded or mixed composition; inverted polarization. Another solution is to use lattice matched semiconductor compounds as materials for the light generation region. However, in pyroelectric materials spontaneous polarization also exists, which is non-zero even in relaxed or lattice-matched layers. For example, the value of spontaneous polarization in nitrides of group III metals is similar to that arising from piezoelectric effect. Several other techniques to reduce piezoelectric polarization are disclosed by Takeuchi et al in U.S. Pat. No. 6,569,704 and by Goetz et al in U.S. Pat. No. 6,630,692.
As it is followed from the above discussion, development of a highly efficient light emitting structure will be more effective in case of consistent solutions of preferably all of the above-mentioned problems.